1. Field of the Invention
The present invention relates to a heat exchanger suitable for use in an air conditioning system for vehicles, and more particularly to improved end portions for heat transfer tubes in heat exchangers.
2. Description of the Related Art
FIGS. 5-7 depict a conventional heat exchanger for use in an air conditioning system for vehicles. In FIGS. 5 and 6, a heat exchanger 21 comprises a pair of tanks 22 and 23. Inlet pipe 24 and outlet pipe 25 are connected to tank 22. A plurality of heat transfer tubes 26 (for example, refrigerant tubes) are fluidly connected between tanks 22 and 23. Each tube 26 has a central portion 26a and end portions 26b having diameters which are smaller than the diameter of central portion 26a. End portions 26b are inserted into holes 22a and 23a disposed in tanks 22 and 23, respectively, and fixed to tanks 22 and 23 by brazing. A partition 27 is provided in tank 22 at a center portion thereof. A heat medium, for example, refrigerant, flows from inlet pipe 24 to outlet pipe 25 through the interior of tank 22, down heat transfer tubes 26, through the interior of tank 23, up heat transfer tubes 26 and through the interior of tank 22, as shown by arrows in FIG. 6.
FIG. 7 depicts the structure of the connection between end portion 26b of each heat transfer tube 26 and tank 22 or 23. End portion 26b extends straight through tank 22 (23) and has a substantially uniform diameter which is smaller than the diameter of central portion 26a. End portion 26b is inserted through hole 22a (23a) and fixed to tank 22 (23) by brazing, welding, gluing or the like between the periphery of end portion 26b and the inner edge of hole 22a (23a). Gap A between the periphery of end portion 26b and the inner edge of hole 22a (23a) generally has a relatively small width, for example, not more than about 0.2 mm (0.008 in), so that a sufficiently thick layer of brazing material may be extended to uniformly cover end portion 26b. Other connecting methods such as gluing or welding may also be used. In such a case, the width of gap A is also relatively small, for example, not more than about 0.2 mm (0.008 in). Specifically, if the width of gap A is greater than about 0.2 mm (0.008 in), it is difficult to provide a sufficient amount of brazing material in the gap to enable proper brazing to occur.
Providing end portions 26b as straight portions, however, presents difficulties in the manufacturing of the heat exchanger. Because the end portions 26b are straight and have a diameter which is only slightly less than the diameter of the holes, it is not easy to insert end portions 26b into holes 22a and 23a. Therefore, this type of heat transfer tube does not permit easy assembly of the heat exchanger.
To solve such problems, another structure for end portions of heat transfer tubes shown in FIG. 8 has been designed. In this structure, a heat transfer tube 31 comprises a central portion 31a and end portions 31b. Each end portion 31b is tapered from central portion 31a, so that the diameter of end portion 31b gradually decreases from a maximum diameter at a base 32 to a minimum diameter at a tip 33. The width of gap A between base 32 and the inner edge of hole 22a (23a) of tank 22 (23) is equal not more than about 0.2 mm (0.008 in). In such a structure, because end portions 31b are tapered, they may be more easily inserted into holes 22a (23a). Therefore, this type of heat transfer tube permits easier assembly of the heat exchangers.
In manufacturing this type of heat exchanger, however, sometimes central portion 31a is not long enough to properly position end portion 31b for brazing, as shown in FIG. 9. This problem often occurs because of warping of tube 31 or nonuniformity of the length of tubes 31 in the direction of the y-axis depicted in FIGS. 6 and 9. When using tubes 31, as shown in FIG. 9, the width of gap A is enlarged by the discrepancy in the incorrect position along the y-axis of the tube end portion 31b. This enlargement of the width of gap A makes effective brazing more difficult. If gap A increases to a width greater than about 0.2 mm, it is difficult to provide enough brazing material to extend around the periphery of end portion 31a. If the width of gap A is much wider than about 0.2 mm (0.008 in), effective brazing becomes impossible. In other words, a tapered end portion allows for only small variations in the positioning of end portion within the hole of the tank due to the length of the tubes.
If straight end portions 26b are used, however, a wider variation in the length of tubes 26 may be allowed. Because the diameter of straight end portion 26b may be substantially uniform, the width of gap A is substantially constant as long as end portion 26b is disposed in hole 22a (23a).